I did see some attempts at building a dielectric polymer speaker, one even with its Class D electrostatic amplifier. These are not near commercialization, but the class D-electrostatic angle is interesting enough considering efficiency alone.
This is part of the reason why I'm getting increasingly unsure about the whole waveguide/horn idea. How can it be good to pressure and massage the soundwaves into a different form right after they're emitted from the driver? One might think, for various reasons, that a waveguide or horn is nevertheless a trade-off that is worth it (directivity, efficiency etc). But it seems intuitively reasonable to me that it would be better to achieve even directivity without the use of waveguides.
Most cones, being less than rigid, should have some midrange effect that is recognizable as horn loading. Horn effects aside, the directivity limitation of conventional designs, ones that are based on rigid pistons, are well known as to be textbook. While material choice can mitigate the presence of higher-order modes from the diaphragm, this limits you to a well-defined pattern.
The BMR can be likened to an extreme case of a carefully damped flexible cone. It yields an extended high-frequency response on-axis from the flat profile. The modes beyond the zero mode, the piston, fill in the response off-axis. The obvious problem with the BMR is that the added masses, as well as the thick membrane, result in low efficiency above Fs, even by the standards of compact drivers. The natural applications are limited to close use and lower SPL, such as a compact speaker.
Scaling up, there is also the distributed mode loudspeaker. AFAIK only sold by Tectonic Labs, as with the BMR. Presents a diffuse sound source and can offer a very wide off-axis response. But being diffuse, it should sound quite different from conventional loudspeakers both in the direct sound and reflections. Most of the DMLs also include ribbon tweeters, which makes me wonder how they fare in the upper octaves.